A replicated study in 1941-6 at a wetland site in Connecticut, USA (Frank 1948), found that wood ducks Aix sponsa nested in only 12 of 274 nest boxes (4%) erected between 3.6 m and 7.3 m off the ground. In contrast, up to 67% of boxes were occupied by grey squirrels Sciurus carolinensis at once, with other rodents and insects also occupying boxes.